var num1 = 1, num2 = 2
function cal () {
  var num1 = 10, num2 = 20
  console.log(this.num1 + this.num2)
}
var calBind = cal.bind({ num1: 100, num2: 200 })

new cal()
cal()
new calBind()
calBind()

Array.prototype.method = function () {
  console.log(this.length)
}
var myArr = [1, 2, 3]
myArr.name = 'array'
myArr.method()
for (let key in myArr) {
  console.log(key)
}
